The PCB is in two parts.

One part with the square plastic black thing (A transformer) provides low voltage DC to the trigger button. This switches the PCB mounted relay that I guess switches mains supply to the welding transformer.

The second part has the potentiometer and transistor thing on it to regulate the wire speed motor.

I've traced out the actual circuit but my camera battery is flat so I'll upload it later today.

What do the two muddy purple and black wires with 1/4" spade terminals go?

Questions?
A] What is the number on the transistor thing.

B] Does anything happen you pull the trigger? ie is the relay activating (you can see as its inside a clear plastic case)